Since their initial detection in 1922, the identification of the carriers of the Diffuse Interstellar Bands (DIBs) has gone on to become one of the longest-standing problems in astrophysics. The DIBs themselves are ubiquitous, intrinsically weak spectroscopic absorption features seen in the spectra of bright background sources (e.g. stars), that arise from the presence of foreground diffuse interstellar material. Although hundreds of DIBs have now been detected in a wide range of sightlines, the identities of their carrier species remain infuriatingly elusive. This review talk will introduce the DIBs, summarize their basic properties and place them in a broader astronomical context, emphasizing precisely what we do and do not know about them. Popular carrier candidates will be critically compared and reviewed. In particular, the role of various extremes of the DIBs world on further constraining them will be emphasized. Such extremes include DIBs along very high and low extinction sightlines, DIBs at long wavelengths and DIB properties seen at ultra-high spectroscopic resolutions.